This invention concerns a method and an apparatus for measuring concentration.
A suspension is a fluidum including at least two components i.e. a liquid and solid particles. With respect to pulp suspensions for paper manufacture, the liquid is substantially water and the solid particles are fibres and possibly fillers, chemicals and impurities.
When a suspension flows through a conduit as created a boundary layer adjacent to the conduit wall, where the flow velocity is low, closest to the conduit wall the velocity is practically zero. For reasons of flow physics, a two-component fluidum such as a suspension will contain more liquid in the boundary layer, meaning that the concentration of solid particles is lower in the boundary layer than in the main flow of the suspension.
For this reason suspension concentration measurements with respect to solid particles has to be made in the main flow or a flow which is representative of the concentration of the main flow. The latter is commonly applicable in respect of pulp suspension measurements after a shut down, when the conduit has been emptied from water and pulp, whereby a dried pulp fibre plug is usually formed, which when restarting the plant affects unprotected instruments inside the conduit, or in respect of unscreened suspension where unsolved pieces brought along with the suspension might damage the gauge and/or permanently affect the result of the measurement.
To this end it is previously known to place the gauge inside a side mounted bulb, where the detector element is placed protected from said possible harmful impurities. However, since the bulb is attached to the conduit wall it will to a great expend be fed with the boundary layer if no particular measures are taken, and therefore a concentration level which is not representative of the main flow will prevail inside the bulb. This condition results in measurement errors resulting in important economic drawbacks. For this reason, devices of this kind has been provided with a propeller which feeds the concentration of the main flow into the bulb and thus to the region of the detector element.
The prior art, for this reason, demands the use of an important pumping effect in order to somehow ensure that the main flow concentration will be brought into the bulb, in particular if great flows are normally present in the conduit, which is the case with respect to pulp suspensions. This results in considerable energy consumption, need for a powerful motor installation and servicing and maintenance for this installation including several devices and moveable parts. Using the prior art is also a risk of the supply from the main flow being insufficient or uneven. Erroneous measurement results can therefore not be excluded.
It is an aim of this invention to avoid the drawbacks of the prior art and to provide a method and a device allowing a reliable and economic solution.
By the boundary layer of the suspension flow, which is closest to the conduit wall, being deflected, it is brought away from the region of the detector element, which in turn is located in the outer area of, or outside the main extension path of the conduit, and this is achieved by a flow modifying element being a fixed deflector, it is ensured that the boundary layer influence on the pulp concentration measurements are eliminated. Contrary to the prior art the deflection is not obtained by an outside power source but instead by the propulsive power imparted to the main flow by the normal propulsive pump.
Since the boundary layer has been deflected in the region of the detector element it will operate in the suspension concentration prevailing in the main flow which gives essentially safer measurement results. It could be mentioned that it is desirable, in the prior art as well as with respect to the invention, to carry out the detection in the suspension conduit such that the flow is disturbed as little as possible in order to avoid unnecessary losses in the flow. I.e. located at the side of the conduit. A careless placement of the detector extending too far into the conduit could also result in plugging of the conduit, resulting from collecting of possibly flowing unsolved objects, beside damages on the detector, as is discussed above.
The feature of a protective bulb, results in enhanced safety for the detector element against damages and unwanted influences and allows reduced effect on the stream in the conduit. By the boundary layer being directed into the bulb, a detector element being arranged in the region of the mouth of the bulb can safely operate in a pulp concentration which is representative of the main flow.
By the deflector being curved in the direction the inside of the bulb, the flow conditions are enhanced, which is accentuated if the intake area of the deflector exceeds the outflow area. The latter results in increased velocity at the entrance of the boundary layer into the bulb, so that its flow inside the bulb is promoted. For this purposes the bulb has suitably been given an even curve so that the boundary layer safely flows along the outside wall of the bulb from the deflector to the xe2x80x9cdownstream sidexe2x80x9d of the bulb. In particular, concerning pulp suspensions, there is normally a relatively high pressure present in the conduit and for this reason the bulb must be constructed to withstand this pressure, which results in that it is usually designed with double-curved surfaces as often as possible.
By arranging a screen upstream from the deflector, it is ensured that possible larger impurities following the stream are brought away from the inlet of the deflector, so as to prevent plugging. The screen is suitably arranged such that the distance between the screen elements are smaller than the distance between the deflector plate and the wall of the conduit and the bulb, thus minimising the risk of plugging. It is preferred that the screen is comprised of repulsive means extending from the conduit wall a distance upstream from the deflector and extending towards the inside of the conduit, e.g. linearly. At the position of the detector they provide the same extension into the conduit as the deflector or an inward extension exceeding that of the deflector. A simple screen may be comprised of a number of triangular plates being distributed along the width of the deflector and having a pointed corner applied against the conduit wall being directed against the flow direction.